MySQL select last row each day - mysql

Question

Trying to select last row each day.
This is my (simplified, more records in actual table) table:
+-----+-----------------------+------+
| id | datetime | temp |
+-----+-----------------------+------+
| 9 | 2017-06-05 23:55:00 | 9.5 |
| 8 | 2017-06-05 23:50:00 | 9.6 |
| 7 | 2017-06-05 23:45:00 | 9.3 |
| 6 | 2017-06-04 23:55:00 | 9.4 |
| 5 | 2017-06-04 23:50:00 | 9.2 |
| 4 | 2017-06-05 23:45:00 | 9.1 |
| 3 | 2017-06-03 23:55:00 | 9.8 |
| 2 | 2017-06-03 23:50:00 | 9.7 |
| 1 | 2017-06-03 23:45:00 | 9.6 |
+-----+-----------------------+------+
I want to select row with id = 9, id = 6 and id = 3.
I have tried this query:
SELECT MAX(datetime) Stamp
, temp
FROM weatherdata
GROUP
BY YEAR(DateTime)
, MONTH(DateTime)
, DAY(DateTime)
order
by datetime desc
limit 10;
But datetime and temp does not match.
Kind Regards

Answer 1

Here's one way, which gets the MAX date per day and then uses it in the INNER query to get the other fields:
SELECT *
FROM test
WHERE `datetime` IN (
SELECT MAX(`datetime`)
FROM test
GROUP BY DATE(`datetime`)
);
Here's the SQL Fiddle.

Answer 2

If your rows are always inserted and never updated, and if id is an autoincrementing primary key, then
SELECT w.*
FROM weatherdata w
JOIN ( SELECT MAX(id) id
FROM weatherdata
GROUP BY DATE(datetime)
) last ON w.id = last.id
will get you what you want. Why? The inner query returns the largest (meaning most recent) id value for each date in weatherdata. This can be very fast indeed, especially if you put an index on the datetime column.
But it's possible the conditions for this to work don't hold. If your datetime column sometimes gets updated to change the date, it's possible that larger id values don't always imply larger datetime values.
In that case you need something like this.
SELECT w.*
FROM weatherdata w
JOIN ( SELECT MAX(datetime) datetime
FROM weatherdata
GROUP BY DATE(datetime)
) last ON w.datetime = last.datetime
Your query doesn't work because it misuses the nasty nonstandard extension to MySQL GROUP BY. Read this: https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html
It should, properly, use the ANY_VALUE() function to highlight the unpredictability of the results. It shoud read ....
SELECT MAX(datetime) Stamp, ANY_VALUE(temp) temp
which means you aren't guaranteed the right row's temp value. Rather, it can return the temp value from any row in each day's grouping.